1. Field of the Invention
The present invention relates to a laser diode module application equipment and more specifically to a technology for increasing an information recording density of an optical disc and to optical communication equipment.
2. Description of the Related Art
For higher optical disc recording speed and recording density, the output of a light source for reading and writing an optical disc needs to be increased or the wavelength of a light source shortened. This raises an optical output per unit volume or area of an interior or edge facet of a resonator in a diode laser, which in turn makes the diode laser more vulnerable to deterioration. This problem applies to conventional infrared and red diode lasers and also to blue diode lasers with a wavelength of 350 nm to 450 nm. This problem becomes more conspicuous as the wavelength decreases as in blue diode lasers. The reason for this is that as the wavelength decreases, the energy of photons increases and is likely to cause damages to crystal, making it difficult to secure a long service life. Conventional practice has involved putting a diode laser in a metal or ceramic package, covering the package with a lid and hermetically or airtightly sealing it to isolate the laser from open air.
Attempts are being made to apply a non-hermetic package using a plastic package. A known non-hermetic sealing technique in a diode laser module for optical fiber communication protects edge facets with transparent resin such as silicone gel by potting to isolate it from external air (K. Tatsuno, et al: IEEE Journal of Lightwave Technology, Vol. 17, No. 7, pp. 1211-1216 (1999)).
The hermetic seal using metals for airtight sealing, however, raises cost and therefore techniques for lowering the production cost are being called for.
The technique for optical fiber communication which protects the end facets of an optical element by potting it with a transparent resin is not yet applied to optical writing and reading.
Application of an end face protection technique, or potting, using a transparent resin for optical fiber communication to the protection of a diode laser used in an optical disc head can prevent degradation caused by exposure to open air. Examinations by the inventors of this invention have found that a simple application of this technique gives rise to a new problem, i.e., an aberration caused by the transparent resin applied over the diode laser. The size of a light spot formed on an optical disc is a wavelength divided by a numerical aperture of an objective lens. This value is a diffraction limit. This condition can be guaranteed only when the aberration of an optical system, which focuses a beam from the diode laser on an optical disc, is removed well.
When a transparent resin is applied over a diode laser, a rough surface differing from one potting job to another is formed on the transparent resin. This causes optical path lengths in the transparent resin that rays emitted from the diode laser travel before going out into air, i.e., the lengths of rays in the resin multiplied by the refractive index, to differ from one another, resulting in an uncontrollable wavefront aberration. When the wavefront aberration is out of an allowable range, a light spot with diffraction limit can no longer be formed on the optical disc, failing to resolve fine pits, minimum units for information recording on an optical disc, in a satisfactory condition.
This is explained by referring to FIG. 1. A beam 101 from a diode laser 100 is focused by an objective lens 102 to form a spot 108 on an optical disc 103. Light reflected from the optical disc 103 passes through the objective lens 102 to reach a beam splitter 109. Light reflected by the beam splitter 109 carries an automatic focus signal, a tracking signal and an information signal and reaches a photodetector 110 where it is subjected to opto-electronic conversion, thus achieving the read/write operation on the optical disc. The diode laser 100 is contained in a package 104 in which it is embedded or potted in a transparent resin 105 for securing reliability of the diode laser. If the surface of the filled resin is uneven or rough, the wavefront 106 generated is distorted and greatly deviates from an ideal spherical wavefront such as indicated at 107. A spot 108 formed on the disc 103 is therefore spread by aberration, failing to resolve fine pits on the disc, which in turn causes errors in a retrieved signal.
The present invention therefore provides means for eliminating adverse effects of the wavefront aberration caused by the transparent resin for encapsulating the diode laser. That is, a transparent parallel-plane plate is arranged between the diode laser and the focusing lens and a space formed between the diode laser and the parallel-plane plate is filled with the transparent resin. With this arrangement, the wavefront aberration can be kept within a controllable range. The word xe2x80x9ctransparentxe2x80x9d in this specification means that the resin has a sufficiently high transmissivity so that a quantity of light that travels from the light source via an optical disc to the photo-detector has, after being opto-electronically converted, a current value high enough to secure a desired signal-to-noise ratio. The transparent resin need only be filled into the optical path of the laser beam between the parallel-plane plate and the laser emitting portion. Other parts than the optical path do not have to be filled with the resin because they have nothing to do with the wavefront aberration. That is, the only requirement is that the filling material should cover the surface of the diode laser including the laser beam emitting portion and fill the laser beam path between the laser beam emitting portion to the parallel-plane plate. This arrangement keeps the surface of the diode laser including the laser beam emitting portion from exposure to open air, thus eliminating drawbacks such as corrosion of the laser and deterioration due to foreign matters adhering to the laser surface.
The following relationship holds between a center intensity xcex7 of a beam spot formed on an optical disc and a standard deviation [RMS] of wavefront aberration that the beam spot forming wavefront has in a pupil of the objective lens:                     η        =                  1          -                                                    (                                                      2                    ⁢                    π                                    λ                                )                            2                        ⁢                                                            [                  RMS                  ]                                2                                                                        (        1        )            
The center intensity xcex7 of the beam spot formed on the optical disc, when xcex7 that produces no aberration is taken as 1, must be 0.8 or higher in order to read pits, minimum recording information units on the optical disc, with a high signal-to-noise ratio. The standard deviation of wavefront aberration for this value is 0.07 wavelength. It is therefore desired that the overall wavefront aberration caused by the diode laser beam source, the filling material, the parallel-plane plate, the objective lens and the optical disc substrate are 0.07 wavelength or less.
When there is a space between the parallel-plane plate and the filling material, the uneven surface of the filling material causes wavefront aberrations. It is therefore important that necessary steps are taken to eliminate such a space.
The filling material, on the other hand, may easily trap moisture and thus it is also important that the space formed between the diode laser and the parallel-plane plate is made non-airtight so that it can communicate with external air to let moisture get out of the filling material.
Further, a space formed by the diode laser beam source, the laser module having a substrate with a mirror for reflecting a beam from the beam source and the parallel-plane plate may be filled with a transparent resin. In that case, when filling the transparent resin, bubbles are likely to be formed easily. To prevent this, a notch may be formed.
When a resin with a higher heat conductivity than that of air is used as the transparent resin, an improved heat dissipating effect can be produced, reducing a thermal resistance between the diode laser and the heat sink. This in turn keeps the operation temperature of the diode laser low and is expected to provide higher reliability.
It is also desired that the refraction index of the filling resin is set higher than unity. The refractive index of larger than 1 improves the refractive index of a space between the diode laser and the mirror and thus narrows the diverging angle of the beam emitted from the diode laser.
Further, when the space between the diode laser and the reflection mirror is filled with a transparent resin, the diverging angle of the diode laser beam is narrowed, which in turn allows the reflection mirror to be reduced in width and thus facilitates the reflection mirror forming process and also the process of mounting the diode laser on the substrate formed with a reflection mirror. The substrate with a reflection mirror is silicone, and by forming on the substrate an array of photo-detectors for focus error detection, tracking error detection and signal detection, the integration level of optical elements can be improved. To ensure that the beam from an optical disc reaches the photo-detectors for automatic focus detection, tracking error detection and signal detection, a diffraction grating or hologram element may be arranged between the optical disc and the substrate.
This diode laser can be used on optical disk read or write drives, or on optical communication apparatus. Optical discs include CD, CD-R, CD-RW, DVD, DVD-R, DVD-RW and ultra-high density optical discs using a blue laser. In addition to read only discs, there are rewritable optical discs using a phase change scheme and a magneto-optical effect. Thus, it is an important technical issue to secure compatibility among a variety of these optical discs. To meet these requirements, the present invention arranges a plurality of diode lasers of different wavelengths in array on the silicone substrate to allow the recording or reading of any of these optical discs with the same optical head.
When diode lasers with long wavelengths, for example 850 nm, 1300 nm and 1550 nm are used, they can be utilized as a light source module for optical fiber communication. In this application, an optical fiber instead of the optical disc is arranged where a light spot is formed. The criterion for evaluating the wavefront aberration with respect to the optical coupling efficiency to the optical fiber is given by equation (1).
After this invention was accomplished, related known examples were surveyed. As a result, JP-A-5-251823 was found. This known example describes that a resin is applied to a diode laser chip and that the resin material is filled as shown in FIG. 3. However, the package is formed with a beam emitting hole at the top and this hole is not mounted with a transparent glass. With this arrangement, the resin surface becomes uneven, resulting in aberrations.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.